Hematite

Hematite is an iron oxide and one of the most common minerals on the Earth's surface. Polished, it looks like a silver-black metal, shiny enough to check your hair in. Yet its name comes from the Greek haima, meaning blood.

The reason shows up when you rub it on a piece of unglazed porcelain: it leaves a red streak. According to Geology.com, this is the most reliable way to identify hematite, because its appearance varies a lot, but the streak is always reddish. Under the metallic shine hides the color of rust, and that is no coincidence: rust is an iron oxide too.

People have used hematite for a very long time. According to Wikipedia, the earliest known use of it as a red pigment dates back about 164,000 years, in the Pinnacle Point caves in South Africa. Some ancient cave paintings were made with red ochre that comes from it. Today it is above all the world's most important iron ore, so there is a good chance that some of the steel around you started out as hematite.

Hematite is found on Mars too. NASA's Opportunity rover found small spheres of it there, which the science team nicknamed "blueberries". According to Geology.com, it is one of the most common minerals in the rocks and soil of Mars, and it adds to the planet's reddish color.

A word for buyers: natural hematite is only weakly magnetic. The "magnetic hematite" sold in shops is usually a synthetic material called hematine. If your beads stick to the fridge, that is probably what you have.